We propose to study the mechanism of renal tubular amino acid reabsorption in normal rabbits and in rabbits exposed to nephrotoxic heavy metals. A method is described, and will be further tested, which permits measurement of the time required by a reabsorbed amino acid or its metabolite to pass from tubular lumen into the peritubular blood capillary in the intact kidney of the living animal. The effects of heavy metals on this reabsorption time, as well as on the Tm and Km of the reabsorptive process, will be determined. Two hypotheses should thus become amenable to testing: 1) transport systems responsible for reabsorption of various classes of amino acids differ from one another by more than the molecular specificity of membrane carriers, and 2) different nephrotoxic heavy metals affect reabsorptive amino acid transport by the same mechanism.